It is generally well known that a trunked communication system may efficiently assign a limited number of communication resources among a plurality of subscriber units. In a typical trunked communication system, a communication resource controller controls, via a control resource, the allocation of the communication resources among a plurality of subscriber units are generally organized into fleets, subfleets or groups, and individuals.
U.S. Pat. No. 4,723,264, issued to Sasuta et al. and assigned to Motorola Inc., is entitled "Signalling Method for Establishing Trunked Communication". It discloses a method for allocating the limited number of communication resources among the subscriber units utilizing only the subscriber unit's individual identification code. To allocate a communication resource, the requesting subscriber unit transmits an inbound signalling word (ISW) to the communication resource controller. The ISW contains the requesting subscriber unit's individual identification code and an operation code. The communication resource controller decodes the ISW and, using a talk-group table, extracts the requesting subscriber unit's talk-group affiliation. After verifying the requesting subscriber unit's talk-group affiliation, the communication resource controller links the requesting subscriber unit to the talk-group wherein, an available communication resource will be allocated to the requesting subscriber unit.
If the requesting subscriber unit's affiliation is not known, the communication resource controller commands the requesting subscriber unit to transmit a talk-group affiliation prior to having access to the trunked system. Also, a subscriber unit can change its talk-group affiliation by transmitting a new talk-group affiliation to the communication resource controller prior to sending an ISW requesting access to a communication resource. This method allows a substantial increase in the number of subscriber units that may access a single trunked system without adding to the control resource traffic congestion.
In the Sasuta patent, each talk-group is preassigned and stored in the communication resource controller and generally organized in a talk-group table. A subscriber unit requesting access to one of the limited number of communication resources is limited to selecting an affiliation with one of the preassigned talk-groups. This, however, may well become a serious limitation when the number of needed talk-groups exceeds the capacity of the talk-group table.
For example, an airport using a trunked communication system employing the techniques of the Sasuta patent may well support only two airlines. Typically, an airline's flights are numbered from 0 to 1,999, thus requiring 2,000 preassigned talk-groups to accommodate their flight schedule. By requiring each flight, or each user-group, to have a preassigned talk-group, a trunked communication system is substantially limited, especially when only a small number of flights would be accessing the trunked communication system at any given time.
One alternative to accommodating a large number of user-groups would be to increase the number of preassignable talk-groups. This alternative would increase the number of talk-groups, or, from the example, would allow the trunked communication system to accommodate more than two airlines; however, this is not without costs. By increasing the number of talk-groups, or adding more bits to the talk-group table, subscriber units designed to operate in conjunction with smaller talk-group tables would not be compatible. Also, this alternative would require more signalling time to send the extra bits, thus adding to the control resource traffic congestion.
Therefore, a need exists for a trunked communication system that provides for a large number of user-groups, without requiring a larger talk-group table, and that does not increase the signalling time of the trunked communication system and does not substantially add to the control resource traffic congestion.